Cast Iron Welding Procedures
Cast Iron is considerably less weldable than low-carbon steel. Cast iron contains much more carbon and silicon than steel, with the result that cast iron is less ductile, and is more metallurgically deformed when welded. However, there have been many successful cast iron repair welds performed in maintenance and casting reclamation applications. The degree of brittleness and likelihood of cracking of the welded material will depend on the type of casting the heat treatment and the welding procedure.
The most important aspect of welding cast iron is to have the surface clean and free of defects prior to welding, since castings that have been in service are likely to be impregnated with oil or grease. All surface contaminants should be removed with solvents, commercial cleaners, or paint removers. Casting skin should be removed from surfaces to be welded. Blind cracks and pits must be completely dressed out to sound metal by mechanical means such as grinding, chipping, rotary filling or shot blasting. Cracks should be excavated to their full length and depth. Excavate spongy areas and pinholes.
Impregnated oil or other volatile matter can be eliminated by using an oxidizing oxy-acetylene flame to heat the casting or weld groove to approximately 900°F for about 15 minutes, and then wire brushing, grinding or rotary filling to remove the residue. This method has the advantage of de-gassing the casting and removing some of the surface graphite as well.
New castings present less of a cleaning problem than castings that have been in service. However, casting skin, sand, and other foreign materials must be removed from the joint to be welded and the adjacent surfaces of the casting.
To repair cracked castings, drill a hole at each end of the crack to prevent it spreading further and grind out to the bottom. Begin welding at the drilled end of the crack, where restraint is greatest and move towards the free end.
Casting which have to transmit fairly heavy working loads often have the weld joint assisted by mechanical means, such as bolt straps, or hoops which are shrunk on. Broken teeth of large cast iron gears are sometimes repaired by studding. Holes are drilled and tapped in the face of the fracture and mild steel studs screwed in. These are then covered with weld metal and build up to the required dimensions. They are machined afterwards or ground to shape.
Precautions When Welding Cast Irons
Factors to consider are the same for all types of cast irons:
- Low ductility with a danger of cracking due to stresses set up by welding. (This is not so important when welding SG iron due to its good ductility.)
- Formation of a hard brittle zone in the weld area. This is caused by rapid cooling of molten metal to form a white cast iron structure in the weld area and makes the weld unsuitable for service where fairly high stresses are met.
- Formation of a hard, brittle weld bead due to pick-up of carbon from the base metal. This does not occur with weld metals which do not form hard carbides such as Monel and high nickel alloys. These are used where machinable welds are desired.
Although much can be done without pre-heating, cracking (due to lack of ductility of castings, especially complicated shapes) may be minimized by suitable pre-heating.
In general all cast irons need to be pre-heated when oxyacetylene welding. This pre-heating reduces the welding heat-input requirements. High pre-heat is needed when using a cast iron filler metal because the weld metal has low ductility near room temperature. To avoid such pre-heating requirements, you may use Aufhauser NickelRod #99, with the base metal at or slightly above room temperature. The weld readily yields during cooling and relieves welding stresses that might otherwise cause cracking in the weld.
- Local preheating occurs where parts not held in restraint may be preheated to about 500°C in the area of the weld, with slow cooling after welding is completed. Cracking from unequal expansion can take place during the preheating of complex castings or when the preheating is confined to a small area of a large casting. This is why local preheating should always be gradual.
- Indirect preheating involves preheat of 200°C for other critical parts of the job in addition to local preheating. This is done so that they will contract with the weld and minimize contraction stresses. Such a technique is suitable for open frames, spokes etc.
- Complete preheating is used for intricate casings, especially those varying in section thicknesses such as cylinder blocks. It involves complete preheating to 500°C followed by slow cooling after welding. The preheating temperature should be maintained during welding. A simple preheating furnace may be made of bricks into which gas jets project. Another may be filled with charcoal which burns slowly and preheats the job evenly.
Post Weld Heating
Post weld heat treatment may consist of either full annealing or stress relieving: when heat treatment is not applied, the welded casting is usually cooled slowly from the welding temperature to room temperature by covering it with insulating material such as lime, ground asbestos, or vermiculite.
Stress relieving at 1150°F and then furnace cooling to at least 700°F is recommended whenever feasible.
Full annealing at 1650°F is sometimes employed to produce greatest softening of the weld zone or a more complete stress relief. However, annealing lowers the as-cast tensile strength of all but the softest irons.
In critical applications that require radiographic or ultrasonic inspection after heat treatment, castings often are inspected before treatment also, to save unnecessary costs if an internal defect should be present.
Satisfactory welds may be made on cast iron without preheating by using electrodes depositing soft metals and peening the weld with a blunt tool (such as a ball hammer) immediately after welding. This spreads the weld metal and counteracts the effects of contraction. Good practice is to deposit short weld runs (50 mm at a time) and then peen before too much cooling takes place. (Aufhauser NickelRod #99 is soft and allows peening).
SMAW of Cast Irons
Grey Cast Iron
NickelRod #99 is more suitable for single layers and for filling small defects as the deposit remains highly machinable. Single-layered welds of NickelRod #55 are not as machinable as NickelRod #99, however they do have increased strength and ductility. NickelRod #55 welds are more tolerant towards contaminants such as sulfur and phosphorous and are superior to NickelRod #99 electrodes when welding casting high in phosphorous.
Peening is a must for grey cast irons.
Joining of cast iron to steel can be performed with either cast NickelRod #55 or NickelRod #99, but NickelRod #55 is preferred. Ferrous based electrodes, including hydrogen controlled types are generally not recommended for welding cast irons. Brackets, lugs and even wear plates can be attached to casting using the correct parameters and
Ductile Cast Iron
Ductile cast iron can only be repaired using NickelRod #55 due to its higher tensile strength and better ductility. When welding ductile cast irons, penetration should be low, and wide joints or cavities should be built up from the sides towards the center. Stringer beads or narrow weaves should be used. Deposit short beads and allow cooling to preheat temperature. Peening is advisable but not as critical as when welding grey cast iron.
Austenitic Cast Irons
These are usually welded with NickelRod #55. Although austenitic castings can be welded with NickelRod #55, the weld may be unsuitable for applications where corrosion/heat resistance qualities do not match the parent metal.
GMAW of Cast Irons
Cast irons are generally considered unweldable using the Gas Metal Arc Welding process.
FCAW of Cast Irons
Flux Cored Arc Welding of cast iron is carried out using higher current than that for Shielded Metal Arc Welding. This is offset by faster travel speeds as for normal Flux Cored Arc Welding. Both grey, ductile and malleable cast irons can be welded using the FCAW process. Preparation and heat treatment are much the same as for SMAW. NickelRod #55 and NickelRod #99 are most suitable for FCAW of cast irons.
Oxyacetylene Welding of Cast Irons
For successful oxy fusion welding, it is essential that the part be pre-heated to a dull, red heat (approximately 650°C). A neutral or slightly reducing flame should be used with welding tips of medium or high flame velocity. The temperatures should be maintained during welding. As with SMAW preparation it is necessary to use a furnace to ensure even heating of large castings. It is important that the casting be protected from draught during welding, and provision should be made to ensure that the required preheat is maintained. It is important to avoid sudden chilling of the casting; otherwise white cast iron may be produced which is very hard and brittle. This may cause cracking or make subsequent matching impossible.
Oxy welding is suitable for grey cast irons with an AWS A5.15 RCI Aufhauser RCI, RCI-A type electrode and should be used with a suitable flux such as Aufhauser Cast Iron Flux.
Austenitic cast irons can only be oxy welded with an AWS RCI-B type consumable.
Braze Welding of Cast Irons
Braze welding should only be used to repair old casting because of the poor color match achieved with newer castings. Braze welding is suitable for grey, austenitic and malleable cast irons. However joint strength equivalent to fusion welds are only possible with grey cast iron. A neutral or slightly oxidizing flame should be used.
Technical and trade information
Braze welding has advantages over oxy welding in that the consumable melts at a lower temperature than the cast iron. This allows lower preheat (320-400°C). As with other forms of welding the surface must be properly cleaned so that carbon doesn't contaminate the weld deposit. The application consumables to use are AWS RBCuZn-C (Aufhauser 681 Low-Fuming Bronze) Types and AWS RBCuZn-D (Aufhauser 773 Nickel Silver) Types.
Brazing of cast irons
Any brazing processes suitable for steel are applicable to cast irons. Pre- and Post- braze operations should be similar to that of a standard brazing processes. Consumables suitable for brazing carbon steel can be used for cast irons.
Powder Spraying of Cast Irons
Powder spraying is particularly suited to edges, corners, shallow cavities and thin sections as there are usually no undercut marks. Porous areas must be ground out to a saucer or cup shape with no overhanging edges. Sharp corners, edges and protruding points must be removed or radiused as they may go into solution in the molten metal causing hard spots.
Spraying and fusing should be as per the normal powder spraying process.
Poor quality or difficult irons can be joined by coating both parts separately with 1-2 mm of spray-fused alloy and then joining the coating together with a suitable nickel SMAW electrode. Consumables are based on a nickel-silicon-boron mixture.
Soldering of cast iron is usually limited to the repair of small surface defects, often sealing areas from leakage of liquid or gases. The casting must be thoroughly cleaned.